Wireless networks are increasingly being used to access the Internet by users. However, wireless networks do suffer from performance problems. A majority of the performance problems in wireless local area networks or Wi-Fi networks occur simply because of the position of stations with respect to the access point. These problems include slow data rate, packet loss during transmission, and loss of access in certain areas within the wireless network range.
One of the well-known problems in Wi-Fi networks is the rate anomaly problem. This problem arises when multiple Wi-Fi stations, which are within the interference range of one another, transmit packets at different transmission rates. The primary reason for the occurrence of this problem is because wireless networks employ IEEE 802.11 protocol, which arbitrates channel access requests on a per-packet basis. Thus, assuming all stations transmit packets of equal size, then the stations or stations that use lower transmission rate consume more ‘airtime’. This limits the throughput of stations that are capable of transmitting at higher rates leading to severe throughput degradation in some cases.
The rate anomaly problem can arise in many situations. In the most common scenario, a station may transmit packets at a lower transmission rate due to the relatively large distance between the station and the access point. This is because the larger distance causes lowering of the signal quality of the transmission and the built-in auto-rate algorithm in the station reduces the transmission rate of the station to compensate for the lower signal quality. However, as discussed above, the station transmitting packets at lower rate can hurt stations that are closer to the access point and capable of transmitting at higher rate.
Another situation where the rate anomaly manifests itself is in a mixed mode operation context. In an example of the mixed mode operation situation, an IEEE 802.11g capable station may co-exist with older IEEE 802.11b stations. An IEEE 802.11b capable station can achieve the maximum rate of 11 Mbps while the highest date rate of IEEE 802.11g capable station is 54 Mbps. Thus, in the above mixed mode operation situation, the IEEE 802.11g capable station may perform suboptimally.